# Energy and Forces in Motion

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Energy and Forces in Motion
Physical Science Chapter 11 and Section 1 of Chapter 13

What is Energy? Energy is the ability to do work.
The Law of Conservation of Energy says: Energy cannot be created or destroyed, it can only change form. An example of energy conservation is when potential energy becomes kinetic energy (and sometimes back into potential energy!)

Potential Energy Potential Energy is the energy an object has because of its position or shape. The object is ready to move, but not yet in motion. Examples: A stretched rubber band, a wind-up toy, a kid at the top of a slide.

Potential Energy Due to Gravity
Gravitational Potential Energy occurs when gravity is the force used to create the potential energy. The more force you build UP against gravity, the greater the GPE. Examples: the low dive vs. the high dive at a swimming pool. Throwing a penny off a chair vs. the Empire State Building!

Chemical Potential Energy
Remember that a chemical change occurs when 2 or more different elements are combined to create a new substance. Chemical potential energy is greatest just before the actual chemical change. Example: The side of the marshmallow is completely brown just before it begins burning!

Kinetic Energy Kinetic energy is the energy of motion. All moving objects have kinetic energy. Kinetic energy depends on speed and mass. The faster the object is moving, the more kinetic energy. Kinetic energy increases as the mass of the object increases.

From Potential to Kinetic
An object’s greatest potential energy is just before it begins to move (winding up a toy). Once the object begins moving, it has kinetic energy. The object has it’s greatest kinetic energy just before it begins to slow down or stop.

Everyday Use of Potential and Kinetic Energy

Forces A force is a push or pull
A net force is the combination of all forces involved. For example, if you and a friend were each pushing a heavy box, then the force would be the combination of the force used by both you and your friend.

Balanced vs Unbalanced Forces
A balanced force means that the opposing forces are the same, so they cancel each other out. When you have a balanced force, you have a net force of 0. An unbalanced force is when the forces are not equal (one is stronger). Net force = greater force – lesser force.

3 Natural Forces On Earth the 3 natural forces are Gravity Friction
Air Resistance (friction of the air)

Centripetal Force Centripetal force is the force needed to move an object in a circle (i.e., an ice skater) Because an object (moving with centripetal force) is always changing direction, it is always accelerating.

Centripetal Force Demo

Gravity and Motion Aristotle believed that the rate an object falls to Earth depends on the object’s mass (the larger the mass, the faster it would fall). Galileo believed that objects fall at the same rate because the rate of gravity is the same (9.8m/s/s). He was correct. It is hard to prove on Earth because of air resistance.

Acceleration at a Constant Rate
the rate of acceleration on Earth is 9.8 m/s2 When an object is dropped on Earth, it is falling at a rate of 9.8 m/s faster than the second before (no matter the size) sec m/s downward 2 sec m/s downward 3 sec m/s downward 4 sec m/s downward And so on…..

Slowing Down Acceleration
Air Resistance is fluid friction, which slows down the acceleration of gravity a force that acts against a falling object. The longer an object falls, the more force of air resistance is built up.

Slowing Down Acceleration
Terminal Velocity - When the force of an object falling and the force of the air resistance pushing up on that object are the same (net force of 0), then the object’s velocity towards the ground will stop accelerating (falls at a constant speed.)

Terminal Velocity Terminal velocity is a good thing. If hailstones didn’t have terminal velocity, they would cause a great deal of harm and damage by the time they hit the ground. Because (most) hailstones are small, their terminal velocity is between 5 m and 40 m/s. If there was no terminal velocity, the hailstones would be hitting us at a velocity of up to 350 m/s!

Terminal Velocity

Creating Terminal Velocity and Free Fall
We use items to help us achieve terminal velocity faster by increasing the force of air resistance. An example would be a parachute. Skydivers say they are in free fall before the parachute opens, but that isn’t correct. Free fall means that there is no other force acting upon the falling object except gravity, and that means you can’t have free fall if there is any air resistance.

Free Fall in Space There is no such thing as weightlessness, even in space. That’s because gravity always exists, and weight is dependent on gravity. When you see astronauts “floating” in space, they still have weight, because there are still objects around you (planets, stars, the space craft). The amount of gravity is so slight, this is why you appear to float.

Free Fall from the Fringes of Space

Orbiting Objects in Free Fall
When the shuttle is orbiting the Earth, it has two motions: - It is traveling forward at a constant speed, It is being pulled by gravity downward towards the Earth. This is called Satellite Motion. The reason why astronauts don’t hit their heads on the ceiling of the shuttle during free fall is because the astronauts are also in free fall towards the Earth.

The Role of Gravity and Orbiting
All orbiting objects move in a circular path (the moon around Earth, the Earth around the Sun, etc.). Any object moving in a circle is constantly changing direction. Any object in motion must be acted upon by an unbalanced force. The unbalanced force that causes objects in orbit to move in a circular motion is called centripetal force.

Projectile Motion and Gravity
Projectile motion is the curved path an object follows when thrown or propelled near the surface of the Earth. Projectile motion can be vertical or horizontal. When they are combined, they form a curved path. The horizontal velocity remains constant, but the vertical velocity slows down because of gravity.

Projectile Motion and Gravity
Example: throw a baseball. When the ball goes forward, it starts to fall to Earth. This motion goes in a curved path along the surface of the Earth. So, if you were to try and hit a bull's-eye with an arrow, where should you aim the arrow before letting go?

Projectile Motion Demo

Friction All moving objects encounter friction, an opposing force to motion Without it most motion would be impossible 4 types, static, sliding, rolling, fluid

Static Friction 1. Normal friction is the outward force from the surface. This creates static force keeps an object from moving (outward force from a surface and bottom of object.) - It is the largest frictional force - Always opposite direction of the applied force (pushing a cart, walking)

2. Sliding Friction Once the object is in motion it experiences sliding friction Opposite direction from applied force Less than static friction so less force is needed to keep it in motion

3. Rolling Friction As something rolls, the object and floor bend slightly. This bend causes rolling friction It is a much smaller force than static friction As much as 1000 times smaller Allows you to move heavy objects Ball bearings reduce friction

4. Fluid Friction It opposes the motion in the LIQUID or GAS
Like swimming, it is hard to move If you are in the air, fluid friction is called air resistance At higher speeds it is very noticeable.

Isaac Newton Remember Newton and the apple? What is a unit of force called? Isaac Newton wrote a book about his observations on motion (Principia). He didn’t actually come up with the official laws, but his findings led to the laws we call Newton’s Laws of Motion.

Newton’s First Law of Motion
“An object at rest remains at rest and an object in motion remains in motion at a constant speed and in a straight line unless acted upon by an unbalanced force.” An unbalanced force doesn’t just mean someone stopping the object. Gravity, air resistance and friction are all unbalanced forces.

Inertia and Mass Inertia is a resistance to change in motion. If there were no gravity, air resistance or friction, then the object would continue to move at the same speed and in the same direction (Earth around the Sun, etc.). The more mass of an object, the greater its inertia (try stopping a car in the same way you stop a bicycle!)

Newton’s 2nd Law of Motion
“The acceleration of an object depends on the mass of the object and the amount of force applied.” Force = mass x acceleration (F = ma) The amount of force depends on the amount of mass and the acceleration rate. If you increase either mass or acceleration, you increase the force. If you decrease one, you will decrease he force. You can make up the difference if you decrease one, but increase the other.

P(momentum) = mass x velocity
Momentum vs Force Momentum is how difficult it is to stop a moving object (so the object is in motion): P(momentum) = mass x velocity Force is how much force the object would have at the moment it collides with another object: F = mass x acceleration

Law of Conservation of Momentum
As with all conservation laws, momentum is not created or destroyed, but is transferred. When you bowl, the momentum of (the rolling bowling ball transfers to the pins.) This law illustrates Newton’s 3rd law. When a moving object strikes another object, the momentum of the moving object (action) transfers and causes the 2nd object to move (reaction).

Newton’s 3rd Law of Motion
“ Whenever one object exerts a force on second object, the second object exerts an equal and opposite force on the first.” Forces work in pairs (action/reaction). Reaction is not always evident on falling objects (such as a bouncing ball). Action/Reaction occurs at the same time!

Normal Force When you stand on the floor, the floor pushes back on your feet. The normal force is the outward force from the surface. The stronger the surface, the more normal force. (Ex: Which would have more normal force: a concrete wall or a wall of marshmallows?)